/*
 * Copyright 2005-2006 Sun Microsystems, Inc.  All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Sun designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Sun in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 */
package com.sun.tools.attach.spi;

import com.sun.tools.attach.AttachNotSupportedException;
import com.sun.tools.attach.VirtualMachine;
import com.sun.tools.attach.VirtualMachineDescriptor;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.ServiceLoader;

/**
 * <p>
 * Attach provider class for attaching to a Java virtual machine.
 * <p/>
 * <p> An attach provider is a concrete subclass of this class that has a
 * zero-argument constructor and implements the abstract methods specified
 * below.
 * </p>
 * <p>
 * An attach provider implementation is typically tied to a Java virtual
 * machine implementation, version, or even mode of operation. That is, a specific
 * provider implementation will typically only be capable of attaching to
 * a specific Java virtual machine implementation or version. For example, Sun's
 * JDK implementation ships with provider implementations that can only attach to
 * Sun's <i>HotSpot</i> virtual machine. In general, if an environment
 * consists of Java virtual machines of different versions and from different
 * vendors then there will be an attach provider implementation for each
 * <i>family</i> of implementations or versions.
 * </p>
 * <p> An attach provider is identified by its {@link #name <i>name</i>} and
 * {@link #type <i>type</i>}. The <i>name</i> is typically, but not required to
 * be, a name that corresponds to the VM vendor. The Sun JDK implementation,
 * for example, ships with attach providers that use the name <i>"sun"</i>. The
 * <i>type</i> typically corresponds to the attach mechanism. For example, an
 * implementation that uses the Doors inter-process communication mechanism
 * might use the type <i>"doors"</i>. The purpose of the name and type is to
 * identify providers in environments where there are multiple providers
 * installed. </p>
 * <p> AttachProvider implementations are loaded and instantiated at the first
 * invocation of the {@link #providers() providers} method. This method
 * attempts to load all provider implementations that are installed on the
 * platform. </p>
 * <p> All of the methods in this class are safe for use by multiple
 * concurrent threads. </p>
 *
 * @since 1.6
 */
public abstract class AttachProvider {
    private static final Object lock = new Object();
    private static List<AttachProvider> providers;
    
    /**
     * Initializes a new instance of this class.
     */
    protected AttachProvider() {}
    
    /**
     * Return this provider's name.
     *
     * @return The name of this provider
     */
    public abstract String name();
    
    /**
     * Return this provider's type.
     *
     * @return The type of this provider
     */
    public abstract String type();
    
    /**
     * <p>
     * Attaches to a Java virtual machine.
     * <p/>
     * <p> A Java virtual machine is identified by an abstract identifier. The
     * nature of this identifier is platform dependent but in many cases it will be the
     * string representation of the process identifier (or pid). </p>
     * <p/>
     * <p>This method parses the identifier and maps the identifier to a Java
     * virtual machine (in an implementation dependent manner). If the identifier
     * cannot be parsed by the provider then an {@link
     * com.sun.tools.attach.AttachNotSupportedException AttachNotSupportedException}
     * is thrown. Once parsed this method attempts to attach to the Java virtual machine.
     * If the provider detects that the identifier corresponds to a Java virtual machine
     * that does not exist, or it corresponds to a Java virtual machine that does not support
     * the attach mechanism implemented by this provider, or it detects that the
     * Java virtual machine is a version to which this provider cannot attach, then
     * an <code>AttachNotSupportedException</code> is thrown. </p>
     *
     * @param id The abstract identifier that identifies the Java virtual machine.
     * @return VirtualMachine representing the target virtual machine.
     * @throws SecurityException           If a security manager has been installed and it denies
     *                                     {@link com.sun.tools.attach.AttachPermission AttachPermission}
     *                                     <tt>("attachVirtualMachine")</tt>, or other permission
     *                                     required by the implementation.
     * @throws AttachNotSupportedException If the identifier cannot be parsed, or it corresponds to
     *                                     to a Java virtual machine that does not exist, or it
     *                                     corresponds to a Java virtual machine which this
     *                                     provider cannot attach.
     * @throws IOException                 If some other I/O error occurs
     * @throws NullPointerException        If <code>id</code> is <code>null</code>
     */
    public abstract VirtualMachine attachVirtualMachine(String id)
        throws AttachNotSupportedException, IOException;
    
    /**
     * <p>
     * Attaches to a Java virtual machine.
     * <p/>
     * <p>
     * A Java virtual machine can be described using a {@link
     * com.sun.tools.attach.VirtualMachineDescriptor VirtualMachineDescriptor}.
     * This method invokes the descriptor's {@link
     * com.sun.tools.attach.VirtualMachineDescriptor#provider() provider()} method
     * to check that it is equal to this provider. It then attempts to attach to the
     * Java virtual machine.
     * </p>
     *
     * @param vmd The virtual machine descriptor
     * @return VirtualMachine representing the target virtual machine.
     * @throws SecurityException           If a security manager has been installed and it denies
     *                                     {@link com.sun.tools.attach.AttachPermission AttachPermission}
     *                                     <tt>("attachVirtualMachine")</tt>, or other permission
     *                                     required by the implementation.
     * @throws AttachNotSupportedException If the descriptor's {@link
     *                                     com.sun.tools.attach.VirtualMachineDescriptor#provider() provider()} method
     *                                     returns a provider that is not this provider, or it does not correspond
     *                                     to a Java virtual machine to which this provider can attach.
     * @throws IOException                 If some other I/O error occurs
     * @throws NullPointerException        If <code>vmd</code> is <code>null</code>
     */
    public VirtualMachine attachVirtualMachine(VirtualMachineDescriptor vmd)
        throws AttachNotSupportedException, IOException {
        if (vmd.provider() != this) {
            throw new AttachNotSupportedException("provider mismatch");
        }
        
        return attachVirtualMachine(vmd.id());
    }
    
    /**
     * <p>
     * Lists the Java virtual machines known to this provider.
     * <p/>
     * <p>
     * This method returns a list of {@link com.sun.tools.attach.VirtualMachineDescriptor} elements.
     * Each <code>VirtualMachineDescriptor</code> describes a Java virtual machine
     * to which this provider can <i>potentially</i> attach.  There isn't any
     * guarantee that invoking {@link #attachVirtualMachine(VirtualMachineDescriptor)
     * attachVirtualMachine} on each descriptor in the list will succeed.
     * </p>
     * @return The list of virtual machine descriptors which describe the
     * Java virtual machines known to this provider (may be empty).
     */
    public abstract List<VirtualMachineDescriptor> listVirtualMachines();
    
    /**
     * <p>
     * Returns a list of the installed attach providers.
     * <p/>
     * <p>An AttachProvider is installed on the platform if:
     * <p/>
     * <ul>
     * <li><p>It is installed in a JAR file that is visible to the defining
     * class loader of the AttachProvider type (usually, but not required
     * to be, the {@link java.lang.ClassLoader#getSystemClassLoader system
     * class loader}).</p></li>
     * <li><p>The JAR file contains a provider configuration named
     * <tt>com.sun.tools.attach.spi.AttachProvider</tt> in the resource directory
     * <tt>META-INF/services</tt>. </p></li>
     * <li><p>The provider configuration file lists the full-qualified class
     * name of the AttachProvider implementation. </p></li>
     * </ul>
     * <p> The format of the provider configuration file is one fully-qualified
     * class name per line. Space and tab characters surrounding each class name,
     * as well as blank lines are ignored. The comment character is
     * <tt>'#'</tt> (<tt>0x23</tt>), and on each line all characters following
     * the first comment character are ignored. The file must be encoded in
     * UTF-8. </p>
     * <p> AttachProvider implementations are loaded and instantiated
     * (using the zero-arg constructor) at the first invocation of this method.
     * The list returned by the first invocation of this method is the list
     * of providers. Subsequent invocations of this method return a list of the same
     * providers. The list is unmodifable.</p>
     *
     * @return A list of the installed attach providers.
     */
    @SuppressWarnings({"Since15"})
    public static List<AttachProvider> providers() {
        synchronized (lock) {
            if (providers == null) {
                providers = new ArrayList<AttachProvider>();
                
                ServiceLoader<AttachProvider> providerLoader =
                    ServiceLoader.load(AttachProvider.class, AttachProvider.class.getClassLoader());
                
                for (AttachProvider aProviderLoader : providerLoader) {
                    try {
                        providers.add(aProviderLoader);
                    } catch (ThreadDeath td) {
                        throw td;
                    } catch (Throwable t) {
                        // Ignore errors and exceptions.
                        System.err.println(t);
                    }
                }
            }
            
            return Collections.unmodifiableList(providers);
        }
    }
}
